The Associated Press just published Mash-up Mania, a good piece covering this space (including a ProgrammableWeb mention). The article hits many of the key trends: heated competition between Microsoft, Yahoo and Google, that most mashups now use Google's platform, and that where it is today is not where it will be soon. As reporter Jonathan Drew notes:
And like many other technology trends that start off in the hands of hobbyists (think blogs), this could turn into something big. Already, many of the creators of the sites are earning pocket change by running ads on their sites. Soon enough, it could be money in the pockets of big corporations...
[Brett Taylor, product manager of Google Local] said Google had no specific plans for advertising on sites that use their maps -- and would probably share revenue with the mash-up creator if they did. But the proliferation of the free, easy to use mapping software has some wondering what's the catch. Chicagocrime.org's founder Adrian Holovaty said he's already created a version of his Web site that can run on Microsoft's mapping service if Google ads ever appear on his site.
"It's in a way a bait and switch because its making this cool free functionality available to anyone who wants it so its sort of hooking people in," Holovaty said. "It's inevitable that Google will start putting advertising on these maps. So they have hundreds of maps on hundreds of decentralized web sites and all of the sudden they're going to have advertising. And I think we'll start to see a backlash when that happens."
There's also a good sidebar on Five things you can do with Mash-ups.